add optional flags argument to pattern decorator

### Clear and concise description of the problem

Currently there is no support for creating a case-insensitive regex pattern. It would be helpful to add an optional flags argument to allow this.

Something like:
```
pattern("(monday|tuesday|wednesday)", "must be a weekday", "i")
```

The third arg is the case-insensitive flag.

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.